A widely used method for the catalytic production of single-wall carbon nanotubes (SWCNTs) is the High Pressure CO (HiPco) disproportionation process, where CO gas and an iron-containing catalyst, Fe(CO)5, are combusted under controlled conditions. Nikolaev, P., Bronikowski, M. J., Bradley, R. K., Rohmund, F., Colbert, D. T., Smith, K. A., “Gas-phase Catalytic Growth of Single-Walled Carbon Nanotubes from Carbon Monoxide,” Chemical Physics Letters 1999, 313: 91. Bronikowski, M. J., Willis, P. A., Colbert, D. T., Smith, K. A., Smalley, R. E., “Gas-phase production of carbon single-walled nanotubes from carbon monoxide via the HiPco process: A parametric study,” Journal of Vacuum Science & Technology, A: Vacuum, Surfaces, and Films 2001, 19 (4, Pt 2): 1800-5. For many material applications of SWCNTs, the carbon-encased iron(0) impurity that results from the HiPco process must be removed with the least damage possible to the SWCNTs.
Oxidative treatment of SWCNTs, typically using O2(g) or other reactive oxidants like HNO3(aq), is widely recommended in literature as a good way to remove carbonaceous impurities and metal catalyst impurities. Vazquez, E., Georgakilas, V., Prato, M., “Microwave-Assisted Purification of HIPCO Carbon Nanotubes,” Chem. Commun. 2002, 20: 2308-9. Xu, Y. Q., Peng, H., Hauge, R. H., and Smalley, R. E., “Controlled Multistep Purification of Single-Walled Carbon Nanotubes,” Nano Lett. 2005, 5(1): 163-8. Rinzler, A. G., Liu, J., Dai, H., Nikolaev, P., Huffman, C. B., Rodriguez-Macias, F. J., “Large Scale Purification of Single Wall Carbon Nanotubes: Process, Product and Characterization,” Appl. Phys. A 1998, 67: 29-37. Chiang, I. W., Brinson, B. E., Huang, A. Y., Willis, P. A., Bronikowski, M. J., Margrave, J. L., et “Purification and Characterization of Single-Wall Carbon Nanotubes,” J. Phys. Chem. B 2001, 105: 1157-61. Kataura, H., Kumazawa, Y., Kojima, N., Maniwa, Y., Umezu, I., Masubuchi, S., “Resonance Raman Scattering of Br2 Doped Single-Walled Carbon Nanotube Bundles,” Molecular Crystals and Liquid Crystals 2000, 340: 757-62. Hou, P. X., Liu, C., Tong, Y., Xu, S. T., Liu, M., Cheng, H. M., “Purification of single-walled carbon nanotubes synthesized by the hydrogen arc-discharge method,” J. of Mater. Res. 2001, 16: 2526-9. Kajiura, H., Tsutsui, S., Huang, H. J., Murakami, Y., “High-quality single-walled carbon nanotubes from arc-produced soot,” Chem. Phys. Lett. 2002, 364: 586-92. Moon, J. M., An, K. H., Lee, Y. H., Park, Y. S., Bae, D. J., Park, G. S., “High-Yield Purification Process of Single-walled Carbon Nanotubes,” J. Phys. Chem. B 2001, 105: 5677-81. Yu, A., Bekyarova, E., Itkis, M. E., Fakhrutdinov, D., Webster, R., Haddon, R. C., “Application of Centrifugation to the Large-Scale Purification of Electric Arc-Produced Single-Walled Carbon Nanotubes,” J. Am. Chem. Soc. 2006; 128(30); 9902-9908. Chiang, I. W., Brinson, B. E., Huang A. Y., Willis, P. A., Bronikowski, M. J., Margrave, J. L., “Purification and Characterization of Single-Wall Carbon Nanotubes (SWCNTs) Obtained from the Gas-Phase Decomposition of CO (HiPco Process),” J. Phys. Chem. B 2001, 105: 8297-8301. While they may effectively remove iron impurities, the main disadvantages of oxidative procedures that employ HNO3(aq) is that the SWCNT material is also significantly damaged, shortened and/or derivatized at the ends of the nanotubes or at defect sites (i.e. with carboxylic acid groups). A non-oxidative acidic treatment (with an acid such as HCl(aq)) is an alternative purification method which does not significantly damage SWCNTs, but it is not as effective at removing other carbonaceous particles and the iron impurity. See Rinzier and Kaijura.
Therefore, it is a desire to provide improved methods for efficiently removing the metal catalyst impurities from carbon nanotubes, while at the same time avoiding derivatization at the ends and at defect sites of the carbon nanotubes.